RNA polymerase is the central enzyme of gene expression and regulation in bacteria. Recently this laboratory has solved the X-ray crystal structure of core RNAP (the processive form of the enzyme) opening a path for subsequent structural studies of this enzyme's interaction with additional subunits and ligands. The overall goal of this proposal is to understand the mechanism of transcription in relation to the structure of the enzyme. Revelation of the holoenzyme structure would add great insight to how the sigma factor allows RNAP to acquire promoter specificity, initiate transcription and regulate gene inhibitors of the enzyme, will increase our current understanding of the transcription mechanisms. These structures are also extremely relevant to medical research for anti-biotic design against bacterial pathogens. This proposal will be accomplished by X-ray crystallography of core RNAP complexed with the following factors: 1) the housekeeping sigma factor (SigA); 2) an initiating nucleotide tri- phosphate, (ATP); 3) a dinucleotide (ApU) and 4) the antibiotics rifampican and streptolydigin.